Further investigation of a factor formed in biological membranes undergoing lipid peroxidation and which appears to be singlet oxygen is proposed. The singlet oxygen-like factor is produced during the break-down of lipid peroxides which are formed in the membrane as a result of free radical production by an NADPH oxidase system bound to that membrane. The free radical-generating activity of this enzyme is inhibited by Vit. E. Vit. E is also a very good O2 quencher. The mechanism of the formation of singlet oxygen will also be investigated, including the influency of the level of dietary unsaturated fat on the process. In addition, since lipid peroxidation and singlet oxygen formation have been implicated in the bactericidal action of leukocytes during phagocytosis, we propose to investigate the burst of NADPH oxidase activity associated with phagocytosis and its relationship to the killing process, with special emphasis on the involvement of lipid peroxidation and apparent singlet oxygen formation. We propose to determine whether or not O2 is actually generated by the leukocytes and if it constitutes part of the killing mechanism. The effect of dietary fat levels and unsaturation and of Vit. E and other antioxidants on these leukocyte phenomena will be determined.